Ultrashort pulse fiber amplifier using rare-earth doped gain fibers

ABSTRACT

Ultrashort pulse fiber amplifier having a pulse width from 200 ps to 200 fs comprising a rare earth oxide doped multicomponent glass fibers for laser amplification, including a core and a cladding, the core comprising at least 2 weight percent glass network modifier selected from BaO, CaO, MgO, ZnO, PbO, K 2 O, Na 2 O, Li 2 O, Y 2 O 3 , or combinations; wherein the mode of the core is guided with step index difference between the core and the cladding, a numerical aperture of the fiber is between 0.01 and 0.04; core diameter is from 25 to 120 micron, and a length of the gain fiber is shorter than 60 cm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a divisional from U.S. patent application Ser. No.14/802,861, filed on Jul. 17, 2015 and now published as US 2016/0218478,which in turn is a continuation-in-part from U.S. patent applicationSer. No. 14/605,740 filed on Jan. 26, 2015 and now patented as U.S. Pat.No. 9,581,760. The disclosure of each of the above-mentioned patentdocuments is incorporated herein by reference.

FIELD OF THE INVENTION

This disclosure relates to ultrashort pulse fiber amplifiers with pulsewidth from 200 ps to 200 fs and comprising rare-earth doped gain fibers.

BACKGROUND

High-power, pulsed fiber lasers are of great interest in applicationssuch as laser micromachining, material processing, nonlinear optics, andlaser sensing. Prior art high power fiber lasers are commonly achievedvia the means of making a fiber-based master-oscillator-power-amplifier(MOPA).

SUMMARY

A 1.01 to 1.12 micron wavelength ultrashort pulse fiber amplifier withpulse width from 200 ps to 200 fs comprising a Ytterbium dopedmulticomponent glass fiber for laser amplification from about 1.01 toabout 1.12 micron wavelength is disclosed. Applicants' Ytterbium dopedmulticomponent glass fiber comprises a core and a cladding. The fiberamplifier does not comprise a pulse stretching or a pulse compressingdevice.

The core glass of Applicants' Ytterbium doped multicomponent glass fibercontains at least 2 weight percent glass network modifier selected fromBaO, CaO, MgO, ZnO, PbO, K₂O, Na₂O, Li₂O, Y₂O₃, or combinations andytterbium oxides from about 3 to about 50 weight percent. The mode ofthe core is guided with step index difference between the core and thecladding, and the numerical aperture of the fiber is between about 0.01and about 0.04. The core diameter is from about 25 to about 60 micron.The length of the gain fiber is shorter than 60 cm.

A 1.51 to 1.65 micron wavelength ultrashort pulse fiber amplifier withpulse width from 200 ps to 200 fs comprising an Erbium dopedmulticomponent glass fiber for laser amplification from 1.51 to 1.65micron wavelength is disclosed. Applicants' Erbium doped multicomponentglass fiber comprises a core, a cladding. The fiber amplifier does notcomprise a pulse stretching or a pulse compressing device.

The core glass of the fiber contains at least 2 weight percent glassnetwork modifier selected from BaO, CaO, MgO, ZnO, PbO, K₂O, Na₂O, Li₂O,Y₂O₃, or combinations and ytterbium oxide from about 0.5 to about 20weight percent. The mode of the core is guided with step indexdifference between the core and the cladding, and the numerical apertureof the fiber is between about 0.01 and about 0.04. The core diameter isfrom about 30 to about 90 microns. The length of the gain fiber isshorter than 60 cm.

A 1.75 to 2.05 micron wavelength ultrashort pulse fiber amplifier withpulse width from 200 ps to 200 fs comprising a Thulium dopedmulticomponent glass fiber for laser amplification from 1.75 to 2.05micron wavelength is disclosed. Applicants' Thulium doped multicomponentglass fiber comprises a core and a cladding. The fiber amplifier doesnot comprise a pulse stretching or a pulse compressing device.

The core glass of the fiber contains at least 2 weight percent glassnetwork modifier selected from BaO, CaO, MgO, ZnO, PbO, K₂O, Na₂O, Li₂O,Y₂O₃, or combinations and ytterbium oxide from about 2 to about 30weight percent. The mode of the core is guided with step indexdifference between the core and the cladding, and the numerical apertureof the fiber is between about 0.01 and about 0.04. The core diameter isfrom about 35 to about 120 micron. The length of the gain fiber isshorter than 60 cm.

A 1.98 to 2.2 micron wavelength ultrashort pulse fiber amplifier withpulse width from 200 ps to 200 fs comprising a Holmium dopedmulticomponent glass fiber for laser amplification from 1.98 to 2.2micron wavelength is disclosed. Applicants' Holmium doped multicomponentglass fiber comprises a core and a cladding. The fiber amplifier doesnot comprise a pulse stretching or a pulse compressing device.

The core glass of the fiber contains at least 2 weight percent glassnetwork modifier selected from BaO, CaO, MgO, ZnO, PbO, K₂O, Na₂O, Li₂O,Y₂O₃, or combinations and ytterbium oxide from about 0.5 to about 20weight percent. The mode of the core is guided with step indexdifference between the core and the cladding, and the numerical apertureof the fiber is between about 0.01 and about 0.04. The core diameter isfrom about 35 to about 120 microns. The length of the gain fiber isshorter than 60 cm.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from a reading of the followingdetailed description taken in conjunction with the drawings in whichlike reference designators are used to designate like elements, and inwhich:

FIG. 1 illustrates a schematic of a prior art fiber-basedmaster-oscillator-power-amplifier (MOPA);

FIG. 2 illustrates a cross section view of Applicants' rare-earth dopedfiber;

FIG. 3 illustrates a cross section view of Applicants' double claddingrare-earth doped fiber; and

FIG. 4 illustrates a cross section view of Applicants' polarizationmaintaining double cladding rare-earth doped fiber.

DETAILED DESCRIPTION

This invention is described in preferred embodiments in the followingdescription with reference to the Figures, in which like numbersrepresent the same or similar elements. Reference throughout thisspecification to “one embodiment,” “an embodiment,” or similar languagemeans that a particular feature, structure, or characteristic describedin connection with the embodiment is included in at least one embodimentof the present invention. Thus, appearances of the phrases “in oneembodiment,” “in an embodiment,” and similar language throughout thisspecification may, but do not necessarily, all refer to the sameembodiment.

The described features, structures, or characteristics of the inventionmay be combined in any suitable manner in one or more embodiments. Inthe following description, numerous specific details are recited toprovide a thorough understanding of embodiments of the invention. Oneskilled in the relevant art will recognize, however, that the inventionmay be practiced without one or more of the specific details, or withother methods, components, materials, and so forth. In other instances,well-known structures, materials, or operations are not shown ordescribed in detail to avoid obscuring aspects of the invention.

High-power, pulsed fiber lasers are of great interest in applicationssuch as laser micromachining, material processing, nonlinear optics, andlaser sensing. High power fiber lasers are commonly achieved via themeans of making a fiber-based master-oscillator-power-amplifier (MOPA).So the fiber amplifier is critical for the laser systems.

FIG. 1 illustrates the schematic of MOPA configuration. The seed laseris amplified by a fiber amplifier. Typically the seed laser is amplifiedby a rare-earth doped gain fiber, which is energized by pump laser.

FIG. 2 shows the cross section view of a rare-earth doped fiber. Thepump laser is combined together with seed laser via the so-called signaland pump combiner. The amplified seed laser can be amplified again inorder to achieve higher pulse energy and higher peak power. When morethan one amplifier is used, the fiber amplifiers are called multi-stageamplifiers. In order to achieve high power, double-cladding rare-earthdoped gain fiber is typically used.

FIG. 3 illustrate the typically cross section view of double claddinggain fiber. The core is used to guide the signal. Here it is called seedlaser. The inner cladding is used to confine the pump lasers. The coreis typically rare-earth doped glass. The rare-earth ion produces gain.For example, ytterbium ion (Yb³⁺) and neodymium (Nd³⁺) offer gain near 1micron wavelength, erbium ion (Er³⁺) produces gain near 1.55 micron,thulium ion (Tm³⁺) and holmium ion (Ho³⁺) can produce gain near 2 micronwavelength.

The inner cladding is typically undoped glass material with a lowerrefractive index in order to form waveguide in the core. The externalcladding layer can be glass material or polymer material, which has alower refractive index to confine the pump laser in the inner cladding.In order to generate polarization maintaining (PM) output, PM gain fiberis needed.

FIG. 4 illustrates the cross section view of a typical PM fiber.

High pulse energy and high peak power is needed for many applications.Due to the strong transverse confinement and long interaction length,power scaling of fiber amplifier is limited by the onset of nonlineareffects.

For single-frequency/narrow-band amplifiers, stimulated Brillouinscattering (SBS) has the lowest threshold and possibly causes much ofthe signal light to be reflected back. For broader signal bandwidth,stimulated Raman scattering (SRS) can happen at higher power levels andtransfer a lot of signal power into unwanted new wavelength components.

The SBS threshold power for narrow band signal is determined by thefollowing equation 1:

$\begin{matrix}{{P_{B\; 0} = \frac{21\;{bA}_{e}}{g_{B}L_{e}}},} & (1)\end{matrix}$where b is a number between 1 and 2 which depends on polarization state.A_(e) is the effective area. g_(B) is the SBS gain coefficient. L_(e) isthe effective transmission length of the fiber.

The threshold power for SRS can be described as the following equation(2)

$\begin{matrix}{P_{R\; 0} = \frac{16\; A_{e}}{g_{R}L_{e}}} & (2)\end{matrix}$where g_(R) is the SRS gain coefficient.

Therefore, the threshold of optical nonlinearity in fiber increases withthe effective area and decreases with the effective transmission lengthof the fiber. The effective area increase with the core diameter of thefiber and the mode filed diameter of the fiber. For single mode core,the mode field diameter is typically proportional to the physical corediameter of the fiber. In order to increase the pulse energy and peakpower of the fiber laser one need to increase the threshold of theoptical nonlinearity of gain fiber. In order to increase the thresholdof the optical nonlinearity of gain fiber, the length of the gain fibershould be short and the core diameter of the gain fiber should be large.

The length of the gain fiber is limited by pump absorption. Claddingpumped fiber amplifiers often have a length of many meters forefficiently absorbing of pump light. A high doping concentration canimprove the absorption and then shorten the length of the gain fiber.However, the doping concentration of typical silica fiber is limited. Sotypically a few meter long gain fiber is used.

The core diameter is limited in order to ensure the fiber is single modefiber. The beam quality will degrade and is no longer single mode whenthe V number of the fiber is more than 2.405,

$\begin{matrix}{V = {\frac{2\pi}{\lambda}a\;{NA}}} & (3)\end{matrix}$where λ is the vacuum wavelength, a is the radius of the fiber core, andNA is the numerical aperture. As can be seen in the equation (3), alower NA value can compensate the increased core size and keep the Vnumber as low as possible.

However there is also a limit to reduce the NA for conventional stepindex fiber. U.S. Pat. No. 8,774,590 disclosed a refractive indexdifference between the core the clad of 0.05 to 0.30% of silica fiber.This patent teaches that a light storing effect of the optical fiberscannot be sufficiently obtained when the relative refractive indexdifference between the core and clad is lower than 0.05%. The refractiveindex of silica glass is approximately 1.45. The refractive index of thecore glass is 1.4507. So the NA of the fiber should be near 0.04 byusing the following equation 4:NA=√n _(core) ² −n _(clad) ²,  (4)Nclad=1.45Ncore=1.45*(1+0.0005)=1.4507

Therefore NA=0.046

When the NA is 0.046, the single mode core diameters are 16.65 micronfor 1 micron wavelength laser, 25.8 micron for 1.55 micron wavelengthlaser, and 33.3 micron for 2 micron wavelength laser in according toequation (3). Although U.S. Pat. No. 8,774,590 claims a core diameter of20 to 30 micron for ytterbium doped fiber laser (ytterbium doped fiberlaser wavelength is 1 micron), the V number is already larger than2.405, which means it is not truly single mode fiber anymore. Fiberbending is needed in order to filter out the higher order mode. So thetrue single mode core diameter near 1 micron is approximately 16.65micron.

Further, silica fibers for U.S. Pat. No. 8,774,590 are formed using MCVD(Modified Chemical Vapor Deposition) or VAD (Vapor Axial Deposition)method to deposit the core material. A problem, however, arises withthese conventional optical fibers in that current optical fibermanufacturing methods are restricted in their ability to preciselycontrol the indices of refraction of the core material (n_(core)) andthe cladding material (n_(clad)). Because of this restricted ability, incommercially practical fiber, the difference between n_(core) andn_(clad) is usually limited by design to no less than 0.1%. This, inturn, restricts the designed size of the core diameter for a givenwavelength, and/or restricts the wavelengths of single-mode operation ofa fiber for a given core diameter.

For example, one common optical fiber manufacturing method referred toas flame hydrolysis uses a burner to fire a combination of metal halideparticles and SiO₂ (called a “soot”) onto a rotating graphite or ceramicmandrel to make the optical fiber perform. See Keiser, Optical FiberCommunications, 2nd ed., McGraw-Hill (1991), which is incorporated byreference herein, at pp. 63-68.

The index of refraction is controlled by controlling the constituents ofthe metal halide vapor stream during the deposition process. The processis “open loop” without a feedback mechanism to precisely control theultimate index of refraction of the optical material. Moreover, themetal halide vapor stream is limited in its controllability and in itsability to control the ultimate index of refraction of the opticalmaterial.

During the process a good portion of the material will be vaporized,therefore, it is extremely difficult to control the difference of therefractive index difference to close to 0.05% (equals to NA of 0.046).So most gain fibers have NA of 0.08 or larger.

Another approach is to use the so-called photonic crystal fiber (PCF)design to achieve a large core diameter. A photonic crystal fiber (alsocalled holey fiber, hole-assisted fiber, microstructure fiber, ormicrostructured fiber) is an optical fiber which obtains its waveguideproperties not from a spatially varying glass composition but from anarrangement of very tiny and closely spaced air holes which go throughthe whole length of fiber. Such air holes can be obtained by using apreform with holes, made e.g. by stacking capillary and/or solid tubesand inserting them into a larger tube. These fibers are not step indexfibers and their guiding mechanism is different from step index fibers.

Laser-active PCFs for fiber lasers and amplifiers can be fabricated,e.g., by using a rare-earth-doped rod as the central element of thepreform assembly. Rare earth dopants (e.g. ytterbium or erbium) tend toincrease the refractive index, the guiding properties are determined bythe photonic microstructure only and not by a conventional-typerefractive index difference. For high-power fiber lasers and amplifiers,double-clad PCFs can be used, where the pump cladding is surrounded byan air cladding region (air-clad fiber). Due to the very large contrastof refractive index, the pump cladding can have a very high numericalaperture (NA), which significantly lowers the requirements on the pumpsource with respect to beam quality and brightness.

Such PCF designs can also have very large mode areas of the fiber corewhile guiding only a single mode for diffraction-limited output, and arethus suitable for very high output powers with excellent beam quality.

But PCF (microstructured fiber) has many disadvantages includingdifficulty for fabrication, difficulty for fusion splicing, poor thermalconductivity of the air-gap, and relatively low doping in the core ofthe fiber. Therefore, it is strongly desired to have a step index fiberwith a large core diameter, which is truly single mode fiber.

We disclose a type of gain fiber, which has a numerical aperture ofbetween 0.01 and 0.04, resulting an extremely large single mode corediameter. Here the host of the rare-earth ions, the gain elements, isthe multicomponent glasses, which is different from the most commonlyused silica glass.

It is well known that silica fibers are made with vapor depositionmethod, which contains almost no alkali metal ions nor alkaline earthmetal ions because these ions are not compatible with vapor depositionprocess. The total content should be less than 0.1 weight percent.Multicomponent glasses always contain alkali metal ions or alkalineearth metal ions, which is at least more than 1 weight percent.

The alkali metals include Lithium (Li), Sodium (Na), Potassium (K), andthe alkaline earth metals are beryllium (Be), magnesium (Mg), calcium(Ca), strontium (Sr), and barium (Ba). These alkali metal ions oralkaline earth metal ions are called glass network modifier inmulticomponent glasses. Other metal ions such as Zn and Pb can act asglass network modifiers, which again is not compatible with vapordeposit process.

Multicomponent glasses include phosphate glasses, silicate glasses,tellurite glasses, germanate glasses, et al. U.S. Pat. No. 6,816,514, inthe name of Jiang disclose rare-earth doped phosphate-glass fiber forfiber laser application. U.S. Pat. No. 6,859,606 in the name of Jiang,disclose erbium doped boro-tellurite glasses for 1.5 micron fiberamplification. U.S. Pat. No. 7,298,768 in the name of Jiang, disclosegermanate glasses for fiber lasers. U.S. Pat. No. 8,121,154 to Jiangdisclosed silicate glasses for fiber laser applications. Multicomponentglass fibers are used for fiber laser application because of theirscapability of high doping concentrations. These patents limit theiradvantages of using a relatively shorter piece of gain fiber compared tosilica glass fiber.

But for high pulse energy fiber lasers, a large core diameter iscritical. Applicants have discovered that a large core diameter can beobtained from multicomponent glass gain fibers. The numerical aperturecan be from 0.01 to 0.04. Therefore, the core diameter can be from 25micron to 60 micron for 1 micron wavelength, 35 micron to 90 micron for1.55 micron wavelength, and 45 micron to 120 micron for 2 micronwavelength.

Applicants doped high rare-earth ions into the fiber, so the totallength of the gain fiber was no longer than 60 cm. Therefore the gainfiber could be packaged straight. No bending was necessary.

Because of the extremely large core diameter and relatively short lengthof gain fiber, a peak power of greater than 50 kW can be achievedwithout optical nonlinearity.

Applicants have developed a new cladding pumped polarization maintainingYb doped fiber based on silicate materials. With large mode size, highYb doping level and low NA, the fiber amplifier has achieved record highthreshold for nonlinear effects while keep excellent diffraction limitedbeam quality. Table 1 compares the parameters of Applicants' Yb-dopedfiber with most popular commercial cladding pumped Yb fibers.

TABLE 1 Fiber Commercial Commercial Applicants' Yb1200-10/125Yb1200-20/125 Yb #35 Core diameter 10 μm 20 μm 30 μm Estimated mode area80 μm² 320 μm² 720 μm² A_(e) NA 0.08 0.08 0.025 V number @ 1064 nm 2.364.74 2.21  Fiber modes Single mode Multimode Single mode Inner cladding125 μm 125 μm 135 μm diameter Doping concentration 1.2 weight 1.2 weight10 weight percent percent percent Nominal cladding ~7 dB/m ~30 dB/m ~500dB/m absorption at 976 nm (For small signal) Fiber length for 7.1 m 1.7m 0.1 m nominal 50 dB absorption at 976 nm Nonlinear threshold P₀ 16 P₀640 P₀

As shown in Table 1, Applicants' fiber Yb #35 has an estimated nonlinearthreshold power ˜640 times higher than that of the commercial fiber.

TABLE 2 compares the SBS/SRS thresholds for different input signalsbetween Applicants' fiber and commercial fibers. The nonlinear thresholdof Applicants' fiber is many times higher than commercial fibers. Thethreshold of Applicants' fiber is always many times higher than thetypical commercial fiber, which means high pulse energy can be achieved.

TABLE 2 Fiber Amplifier Commercial Commercial Yb1200- Yb1200-Applicants' 10/125 20/125 Yb #35 7.1 m 1.7 m 0.1 m SBS threshold for 5.9W 94 W 3.7 kW narrow band CW signal SRS threshold for 2.8 kW 45 kW 1.8MW CW signal SRS threshold 280 μJ 4.5 mJ 180 mJ energy for 100 ns pulseSRS threshold 28 μJ 450 μJ 18 mJ energy for 10 ns pulse SRS threshold2.8 μJ 45 μJ 1.8 mJ energy for 1 ns pulse

Picosecond and femtosecond lasers are called ultrashort pulse lasers. Inamplifiers for ultrashort optical pulses, the optical peak intensitiescan be very high, so that detrimental nonlinear pulse distortion or evendestruction of the gain medium or of some other optical element mayoccur. This can be effectively prevented by employing the method ofchirped-pulse amplification (CPA). Before passing through the amplifiermedium, the pulses are chirped and temporally stretched to a much longerduration by means of a strongly dispersive element (the stretcher, e.g.a grating pair or a long fiber). This is called a pulse stretcher. Thepulse stretcher effectively reduces the peak power to a lower levelbecause of the long pulse width, therefore the above-mentioneddetrimental nonlinear effects in the amplifiers can be avoided. Due tothe inherently high nonlinearity of long fibers in fiber lasers andfiber amplifiers, CPA typically is applied for relatively low pulseenergies fiber amplifiers.

After the amplifiers, a dispersive compressor is used, i.e., an elementwith opposite dispersion (typically a grating pair), which removes thechirp and temporally compresses the pulses to a duration similar to theinput pulse duration. This is called a pulse compressor. After the pulsecompression, the peak power of the amplified pulse becomes very high.

The fiber laser system disclosed in U.S. Pat. No. 5,499,134 relied uponchirped fiber Bragg gratings for pulse stretching. U.S. Pat. No.8,503,069 B2 to Martin E. Fermann et al disclose the design ofultra-compact high energy chirped pulse amplification systems based onlinearly or nonlinearly chirped fiber grating pulse stretchers andphotonic crystal fiber pulse compressors. Photonic crystal fiber pulsestretchers and photonic crystal fiber compressors can be implemented.U.S. Pat. No. 8,659,821 B2 to Schimpf et al. disclose a particulardesign for amplifying a stretched pulse and pulse compressing of fiberlaser systems.

Although those stretching and compressing techniques and devices canproduce the high pulse energy and high peak power, the fiber lasersystem is complicated. That is one of the major reasons that ultrashortfiber laser systems are expensive.

In order to avoid pulse stretching, a linearly chirped parabolic shapedpulse is developed for pulse amplification. In general, there are twomethods of parabolic pulse generation available in the fibers.

A first method includes parabolic pulse generation in the fiberamplifiers. This method uses an amplifying medium which has a broadenough gain bandwidth to support an asymptotic self-similar pulsepropagation. A second method is based on passive fibers. For both cases,in the presence of normal dispersion, asymptotic self-similar pulsepropagation can be achieved when the seed pulses with a bandwidthsufficiently smaller than the gain bandwidth propagate in the amplifierwith negligible gain saturation. In such systems, Self-phase modulation(SPM) induced linear chirp can be preserved during amplification withoutsuffering spectral and temporal intensity profile distortion. Otherwise,the bandwidth limitation must be overcome by means of active spectralphase control to compensate undesired higher order phase distortion.

Parabolic pulse formation in highly nonlinear fiber amplifiers can beeffectively described by the modified nonlinear Schrodinger equation:

$\begin{matrix}{\frac{\partial A}{\partial z} = {{{- \frac{i}{2}}\left( {\beta_{2} + {i\;\frac{g}{\Omega_{g}^{2}}}} \right)\frac{\partial^{2}A}{\partial t^{2}}} + {i\;\gamma{A^{2}}A} + {\frac{1}{2}\left( {g - \alpha} \right)A}}} & (1)\end{matrix}$where g and α are the intensity gain and loss per unit length, Ω_(g) isthe gain bandwidth, and β₂ is the group velocity dispersion (GVD).Self-phase modulation (SPM) is governed by the nonlinearity parameterγ=2πn₂/λF, where n₂ is the nonlinear refractive index and λ is thecenter wavelength and F is effective core area in the fiber amplifier.

Parabolic pulse generation and propagation is possible only when theinput pulse has sufficient enough energy to create SPM to accumulatequadratic phase through the normal dispersive media. In other words, itis clear that the mutual balance between normal dispersion andnonlinearity will support the parabolic temporal profile through thepulse propagation in the fiber. Ultimately, this parabolic amplificationstrongly dependent on fiber engineering. So this process is complicatedto implement.

Applicants disclose the ultrashort pulse amplification without pulsestretching and compressing devices by using an extremely large corediameter and a very short length rare-earth doped gain fibers. Thecombination of an extremely large core diameter and a very short lengthgain fiber can effectively suppress the optical nonlinearity. Ultrashortpulse seed laser with pulse with from 200 ps to 200 fs can be amplifiedto more than 1 μJ pulse energy and more than 200 kW.

While the preferred embodiments of the present invention have beenillustrated in detail, it should be apparent that modifications andadaptations to those embodiments may occur to one skilled in the artwithout departing from the scope of the present invention.

We claim:
 1. A 1.51 to 1.65 micron wavelength ultrashort pulse fiberamplifier with pulse width from 200 ps to 200 fs, comprising an Erbiumdoped multicomponent glass gain fiber for laser amplification from 1.51to 1.65 micron wavelength, comprising: a core; and a cladding; wherein:said core comprises at least 2 weight percent glass network modifierselected from BaO, CaO, MgO, ZnO, PbO, K₂O, Na₂O, Li₂O, Y₂O₃, orcombinations thereof; and erbium oxide at a level from about 0.5 toabout 20 weight percent; wherein: a step index difference between thecore and the cladding is configured to guide a mode of the core; anumerical aperture of the gain fiber is between 0.01 and 0.04; a corediameter is from about 30 microns to about 90 microns; a length of thegain fiber is shorter than 60 cm; said fiber amplifier comprising nopulse-stretching devices and no pulse-compressing devices.
 2. The fiberamplifier of claim 1, wherein said erbium oxide is present at a levelfrom about 1 to about 5 weight percent.
 3. The fiber amplifier of claim1, wherein the core diameter is from about 35 microns to about 60microns.
 4. The fiber amplifier of claim 1, wherein the length of thegain fiber is from about 4 cm to about 45 cm.
 5. The fiber amplifier ofclaim 1, wherein the gain fiber includes a polarization maintainingfiber.